Annealing Conditions for Carbon Steel Seamless Pipes

Carbon steel seamless pipe is a widely used pipe material. Due to their high strength and stable mechanical properties, they are widely used in petrochemical, construction, aerospace, manufacturing and other fields. In order to further improve the performance and quality of carbon steel seamless pipes, annealing treatment is often required. Annealing treatment is a heat treatment method that can change the crystal structure and properties of metal materials by slowly increasing the temperature and cooling after reaching a certain temperature.

The annealing conditions of carbon steel seamless pipes are a key issue that directly affects the quality and performance of the product. The following are the relevant return conditions:

Temperature conditions

1. Full annealing

Full annealing is to heat the carbon steel seamless pipe to 30-50℃ above the critical temperature (Ac3). For carbon steel with a carbon content of 0.3%-0.6%, the Ac3 temperature is roughly 750-800℃, which means that the full annealing temperature is usually 800-850℃. At this temperature, all ferrite and cementite in the steel are recrystallized, the grains are refined, and the structural defects are eliminated. For example, in actual production, the hardness of the steel pipe can be reduced by complete annealing, thereby improving the cutting performance.

2. Incomplete annealing

The incomplete annealing temperature is above the critical temperature (Ac1) and below Ac3. Taking hypoeutectoid carbon steel as an example, the Ac1 temperature is about 727℃, and the incomplete annealing temperature can be set at 740-770℃. This annealing method is mainly used to refine the grains and reduce the hardness while maintaining a certain strength and toughness. It can improve the internal structure of the steel pipe to a certain extent, laying the foundation for subsequent processing.

3. Spheroidizing annealing

Spheroidizing annealing is mainly for eutectoid and hypereutectoid carbon steels, and its temperature is generally 20-30℃ above Ac1. For example, for T10 steel with a carbon content of about 1.0%, the Ac1 temperature is about 730℃, and the spheroidizing annealing temperature can be controlled at 750-760℃. The purpose of spheroidizing annealing is to spheroidize cementite, reduce hardness, improve cutting performance, and prepare for subsequent heat treatments such as quenching.

Holding time

The holding time refers to a process in which the pipe is kept at this temperature for a certain period of time after reaching the annealing temperature so that it can reach a uniform grain size. The length of the holding time is also determined by the specific material, wall thickness and requirements of the product. Generally speaking, the holding time is generally about 2-4 hours.

Cooling method

1. Complete annealing

After complete annealing, the cooling speed should be slow, generally cooling to 500-600℃ with the furnace and then air cooling out of the furnace. Such slow cooling can fully decompose the austenite during the cooling process, forming a balanced ferrite and pearlite structure, and avoiding the generation of large internal stress. For example, during the cooling process, the stress inside the steel pipe is released due to the uniform temperature drop, thus ensuring the quality of the steel pipe.

2. Incomplete annealing

The cooling rate after incomplete annealing is also relatively slow, and furnace cooling or burying in sand cooling is usually adopted. The furnace cooling rate can be controlled at 30 - 50℃/h until it cools to a lower temperature. This cooling method can make the organizational transformation more uniform and reduce internal stress. Through slow cooling, the internal structure of the steel pipe gradually stabilizes, providing good conditions for subsequent processing.

3. Spheroidizing annealing

The cooling rate after spheroidizing annealing also needs to be controlled. Generally, after the end of the insulation, it is cooled to 500 - 600℃ at a rate of 10 - 20℃/h, and then air-cooled out of the furnace. This cooling method helps to maintain the spheroidization effect of cementite, so that the steel pipe can obtain good mechanical properties and cutting performance.

Atmosphere conditions in the furnace

During the annealing process, the atmosphere in the furnace is crucial. Generally, a neutral atmosphere or a slightly oxidizing atmosphere is used. If the reducing atmosphere in the furnace is too strong, it may cause decarburization on the surface of the steel pipe, affecting its surface quality and performance. For example, when annealing in a gas furnace, the ratio of gas and air needs to be controlled to avoid producing too much reducing gas. At the same time, too much oxidizing atmosphere will cause the surface of the steel pipe to oxidize and form oxide scale, so the atmosphere in the furnace should be adjusted reasonably.

Furnace loading method

Carbon steel seamless pipes should be placed evenly when loading the furnace to avoid mutual squeezing to ensure uniform temperature in the furnace. Too dense loading will cause uneven heating of the steel pipe and affect the annealing effect. Generally, layered loading or interval loading is adopted. The loading position is reasonably arranged according to the size and shape of the steel pipe so that each steel pipe can be fully heated and cooled.

Summary

Therefore, the selection of annealing conditions for carbon steel seamless pipes is of great significance to the quality and performance of the product. The temperature, insulation time, cooling method and atmosphere must be reasonably selected according to the specific product requirements to ensure that the product meets the required performance and quality requirements.